1. Field of the Invention
This disclosure is related to the field of polymer interlayers for multiple layer panels and multiple layer panels having at least one polymer interlayer sheet. Specifically, this disclosure is related to the field of polymer interlayers comprising blends of two or more resins.
2. Description of Related Art
Multiple layer panels are generally panels comprised of two sheets of a substrate (such as, but not limited to, glass, polyester, polyacrylate, or polycarbonate) with one or more polymer interlayers sandwiched therebetween. The laminated multiple layer glass panels are commonly utilized in architectural window applications and in the windows of motor vehicles and airplanes, and in photovoltaic solar panels. The first two applications are commonly referred to as laminated safety glass. The main function of the interlayer in the laminated safety glass is to absorb energy resulting from impact or force applied to the glass, to keep the layers of glass bonded even when the force is applied and the glass is broken, and to prevent the glass from breaking up into sharp pieces. Additionally, the interlayer may also give the glass a much higher sound insulation rating, reduce UV and/or IR light transmission, and enhance the aesthetic appeal of the associated window. In regard to the photovoltaic applications, the main function of the interlayer is to encapsulate the photovoltaic solar panels which are used to generate and supply electricity in commercial and residential applications.
The interlayer may be a single (or monolithic) layer, a combination of more than one single layer, a multilayer that has been coextruded, a combination of at least one single layer and at least one multilayer, or a combination of multilayer sheets.
In order to achieve the desired and optimal sound insulation for the glass panel, while retaining the impact performance and the optical quality necessary, it has become common practice to utilize multilayered interlayers with at least one soft “core” layer sandwiched between two more stiff or rigid “skin” layers. These layers of the interlayer are generally produced by mixing a polymer resin such as poly(vinyl butyral) with one or more plasticizers and melt processing the mix into a sheet by any applicable process or method known to one of skill in the art, including, but not limited to, extrusion, with the layers being combined by processes such as co-extrusion and lamination. Other additional ingredients may optionally be added for various other purposes. After the interlayer sheet is formed, it is typically collected and rolled for transportation and storage and for later use in the multiple layer glass panel, as discussed below.
Single or monolithic polymer interlayers having improved acoustic properties have been produced previously. One method of producing monolithic or single layer interlayers having acoustic properties is by mixing either a single PVB resin having low residual hydroxyl content (% PVOH), such as 17% or less, with higher amounts of a plasticizer, such as triethylene glycol di-(2-ethylhexanoate) (3GEH), and extruding the mixture to form a polymer interlayer. Alternatively, monolithic or single layer interlayers having acoustic properties can be produced by mixing a single PVB resin having high residual hydroxyl content, such as 18% or higher, with a high amount of a plasticizer or a mixture of plasticizers in which at least one plasticizer is more efficient in plasticizing PVB resin than conventional plasticizer (such as 3GEH). The former method is a more preferred approach. The resultant polymer interlayer having acoustic properties typically exhibits a glass transition temperature, Tg, of 25° C. or lower.
Interlayers with low glass transition temperatures are known to have better acoustic damping performance. Polymer interlayers having lower glass transition temperatures are generally softer, therefore multiple layer glass panels or other laminates made with these softer polymer interlayers exhibit maximum impact penetration resistance at a temperature significantly lower than ambient temperature (i.e., 23° C.). Because of this, thicker polymer interlayers are often required to satisfy the level of impact resistance required in many applications. This softer polymer interlayer, while it has good acoustic properties, is also difficult to manufacture and laminate due to its performance properties.
Multilayer interlayers such as a trilayer interlayer having a softer acoustic dampening core layer and two stiffer skin layers (which provide for improved handling of the interlayer compared to soft monolithic interlayers or interlayers having softer skin layers) are commercially available. The trilayer interlayer is typically produced by encapsulating the soft monolithic acoustic layer with two stiff skin layers through a co-extrusion process. The stiff skin layers typically exhibit a glass transition temperature, Tg, of about 30° C. or more, and the soft acoustic damping core layer typically has a Tg of less than 25° C. While the trilayer interlayer having stiff skin layers has improved handling and processing performance compared to the soft monolayer or monolithic interlayers, these multilayer interlayers are also more expensive to produce than single layer or monolithic interlayers.
Because of the presence of the soft acoustic core layer there are inherent defects with the multiple layer interlayers. One inherent defect in multiple layer interlayers is mottle, which is present in the manufacture of multilayer laminate glass panels having multiple layer interlayers in the final unitary structure. Mottle is an objectionable form of optical distortion or visual defect appearing as uneven spots, or texture. Mottle is caused by small scale surface variations at the interfaces between the soft and stiff layers wherein the individual layers (or the soft and stiff layers) have different refractive indices. Other inherent defects in multiple layer interlayers are bubbles or iceflowers (also known as snowflakes) that develop in the soft core layer in the manufacture of multilayer laminate glass panels, such as in windshields installed in vehicles or in the windows of buildings. Iceflowers are undesirable optical defects which generally are initiated from bubbles at high temperature that expand and branch in radial directions where resistance to the radial expansion is small. The softer core layer of a trilayer interlayer has low resistance to the bubble nucleation and is in favor of bubble nucleation and iceflower formation.
The use of a single or monolithic polymer interlayer in a multiple layer glass panel can eliminate the presence of mottle caused by the variations of the surface at the interfaces of the layers in the multiple layer acoustic interlayers since there is only one layer (and therefore no interfaces between layers). The monolithic polymer interlayer in a multiple layer glass panel can also eliminate the formation of iceflowers and other undesirable optical defects. But as previously discussed, a monolithic interlayer having good acoustic properties can be difficult to manufacture and laminate into a multiple layer glass panel.
In addition to mottle and iceflowers, clarity of the multiple layer panel is another important optical quality, whether or not the polymer interlayer provides sound insulation for the multiple layer panel, or whether or not the polymer is a multilayer interlayer or a monolithic interlayer. Clarity is determined by measuring the level of haze in the multiple layer panel, as further described below. The level of haze must be very low so that the multiple layer panel is clear.
In the manufacturing of polymer interlayers, it has become common practice to recycle a certain amount of the interlayer materials (such as off grade material or trim) which would otherwise be unusable and disposed of at a cost, such as land filled. This practice of recycling material has often resulted in polymer interlayers that have high haze and low visible light transmittance. The high haze or low clarity is caused by the differences in the polymers and/or plasticizers that are blended or mixed together, which causes the light to scatter in the blend or mixture in which there is a sufficiently large difference in the refractive index between the polymers or plasticizers. Accordingly, there is a need in the art for the development of an interlayer, especially a monolithic interlayer, that contains a blend or mix of polymers having different compositions and/or plasticizers while also having a high level of visible transmittance and very low haze.
It is now common to use a multilayer interlayer (such as a trilayer interlayer) to provide high performance laminates, particularly in laminates having improved acoustic performance properties. As previously discussed, the use of multilayer interlayers, however, very often results in an increased level of optical defect problems, such as mottle and haze, as well as other types of performance defects, such as iceflowers, and multilayer interlayers are more expensive to manufacture. As previously discussed, multilayer interlayers such as a trilayer interlayer having stiff skin layers typically exhibit a glass transition temperature, Tg, of about 30° C. or higher in the stiffer skin layers and a Tg of less than 25° C. in the softer, acoustic damping core layer. The stiff skin layers typically contain a resin having a higher level of residual hydroxyl groups, and the soft core layer(s) typically contains a resin having a lower level of residual hydroxyl groups. The higher Tg of the stiff skin layer(s) provides such mechanical properties as impact, as well as improved handling and processing performance, while the softer core layer(s) provides acoustic damping performance.
Because monolithic interlayers having a low glass transition temperature and good acoustic performance are difficult to manufacture and laminate, there is a further need in the art for the development of a monolithic interlayer that has both good acoustic performance and improved handling and processing performance. It is advantageous to produce a monolithic interlayer containing two (or more) resins and a plasticizer, in which at least one resin has a lower level of residual hydroxyl groups and provides good acoustic performance, and at least one other resin has a higher level of residual hydroxyl groups and provides mechanical properties such as impact as well as improved handling and processing performance.